In what circumstances do SPNs get created automatically?

I'm trying to solve some mystery in two of my environments. Both are SQL 2019 deployments, however one has Kerberos configured (i.e. SPNs are registered for the instance and port), while the other deployment does not. I've been trying to investigate how this happened but not got to the bottom of it yet. Basically, I would like to understand the requirement for future deployments. Am I supposed to manually create (or request to be created) the SPNs or there is some way in which they get automatically created? For example, could they have been created in the course of building the application tier? I'd appreciate if anyone is able to throw some light on this for me please. Of course I have been researching all about Kerberos authentication and SPNs but if there are any specific articles on this specific question, I'd appreciate a link to it please. Thank you